This was previously used to allow to save a state in a SPIDriver so we
could synchronize the initialization of AP_Compass and
AP_InertialSensor.
It was only used by MPU9250 and is not used anymore since the move to
AuxiliaryBus initialization and it's not used anymore since c3dae6f
("AP_InertialSensor: MPU9250: Remove methods not used anymore")
The argument in init() is not used by any implementation. Add a second
method without it so the HAL implementation can used it instead. Later
the unused method will be removed.